A frame conventionally has rims, each presenting an inside bezel forming a groove. Under such circumstances, edging a lens for it to be mounted in the frame selected by the future user consists in modifying the outline of the lens to match it to the selected frame. Edging comprises edging for shaping the periphery of the lens and also for appropriately beveling the edge face of the lens.
Edging consists in removing the superfluous peripheral portion of the ophthalmic lens in question so as to reduce its outline, which is usually initially circular, to the outline desired for the surround of the eyeglass in question. This edging operation is usually followed by a chamfering operation, which consists in rounding or chamfering the two sharp edges of the edged lens.
Chamfering is accompanied or preceded by beveling which consists in forming a rib, commonly referred to a bevel and generally of triangular cross-section, on the edge Lace of the ophthalmic lens. The bevel is for engaging in the bezel formed in the surround of the eyeglasses frame in which the lens is to be mounted.
Conventionally, such edger means are constituted by a machine tool, referred to as a grinder, that possesses a main grindwheel set and means for blocking and rotating the lens, said means being constituted by two rotary shafts on a common axis and mounted to move axially relative to each other in order to clamp the lens on its axis with a pinching movement. To enable the lens to move towards or away from the grindwheel during machining, the clamping and drive shafts are carried on a rocker that is movable (in pivoting or translation) transversely relative to the shaft.
Usually, the operations of edging, chamfering, and beveling are performed successively on the same grinder, which is fitted with a suitable set of grindwheels.
At present, it is known that the periphery of an ophthalmic lens can be edged in two steps. A roughing step is performed initially, in which the periphery of the lens is roughed out on a cylindrical roughing grindwheel forming one of the set of grindwheels. During this roughing, the lens is roughed out so as to come close to the shape that is it desired to give to the lens. Thereafter, a finishing step is performed on the periphery of the lens, using a cylindrical finishing grindwheel that is also present in the set of grindwheels, having the same diameter and the same axis as the roughing grindwheel, but possessing grains of a finer size than the grains used in the roughing grindwheel. During this finishing step, less material is removed from the periphery of the lens, and the desired shape is reached with accuracy better than the accuracy that can be obtained with a roughing grindwheel. The finishing grindwheel presents a beveling groove in its edging face so beveling is performed during the finishing step.
Nevertheless, with lenses that are strongly cambered, it is found that a phenomenon occurs whereby the bevel on the strongly cambered portions of the edge face of the lens becomes thinned. This bevel-thinning phenomenon degrades the quality and the precision with which the lens can be mounted in its frame.